This invention relates broadly to an improvement in nuclear fuel elements for use in the core of nuclear fission reactors, and more particularly to improved nuclear fuel elements having disposed therein an additive of a barium-containing material positioned in the plenum of the fuel element and capable of collecting gases through chemical reaction or adsorption.
Nuclear reactors are presently being designed, constructed and operated in which the nuclear fuel is contained in fuel elements which may have various geometric shapes, such as plates, tubes, or rods. The fuel material is usually enclosed in a corrosion-resistant, non-reactive, heat conductive container or cladding. The elements are assembled together in a lattice at fixed distances from each other in a coolant flow channel or region forming a fuel assembly, and sufficient fuel assemblies are combined to form the nuclear fission chain reacting assembly or reactor core capable of a self-sustained fission reaction. The core in turn is enclosed within a reactor vessel through which a coolant is passed.
The cladding serves two primary purposes: first, to prevent contact and chemical reactions between the nuclear fuel and either the coolant, or moderator if present, or both; and second, to prevent the radioactive fission products, some of which are gases, from being released from the fuel into the coolant, or moderator if present, or both. Common cladding materials are stainless steel alloys, aluminum and its alloys and zirconium and its alloys. The failure of the cladding, due to the build-up of gas pressure or other reasons, can contaminate the coolant or moderator and the associated systems with radioactive long-lived products to a degree which interferes with plant operation.
Problems have been encountered in the manufacture and in the operation of nuclear fuel elements which employ certain metals and alloys as the clad material due to the reactivity of these materials under certain circumstances. Zirconium and its alloys, under normal circumstances, are excellent materials as a nuclear fuel cladding since they have low neutron absorption cross sections and at temperatures below about 600.degree. F are extremely stable and non-reactive in the presence of demineralized water or steam which are commonly used as reactor coolants and moderators. Within the confines of a sealed fuel rod, however, the hydrogen gas generated by the slow reaction between the cladding and residual water may build up to levels which under certain conditions can result in localized hydriding of the alloy with concurrent deterioration in the mechanical properties of the cladding. The cladding is also adversely affected by such gases as oxygen, nitrogen, carbon monoxide and carbon dioxide at reactor operating temperatures.
The zirconium cladding of a nuclear fuel element is exposed to one or more of the gases given above during irradiation in a nuclear reactor in spite of the fact that these gases may not be present in the reactor coolant or moderator, and further may have been excluded as far as possible from the ambient atmosphere during manufacture of the cladding and the fuel element. Sintered refractory and ceramic compositions, such as uranium dioxide and others used as nuclear fuel, release measurable quantities of the aforementioned gases upon heating, such as during fuel element manufacture and especially during irradiation. Particulate refractory and ceramic compositions, such as uranium dioxide powder and other powders used as nuclear fuel, have been known to release even larger quantities of the aforementioned gases during irradiation. These gases react with the zirconium clad material containing the nuclear fuel. This reaction can result in the embrittlement of the cladding which endangers the integrity of the fuel element. Although water and water vapor may not react directly to produce this result, at high temperatures water vapor does react with zirconium and zirconium alloys to produce hydrogen and this gas further reacts locally with the zirconium and zirconium alloys to cause embrittlement. These undesirable results are exaggerated by the release of these residual gases within the sealed metal clad fuel element since it increases the internal pressure within the element and thus introduces stresses in the presence of corrosive conditions not anticipated in the original design of the clad tube.
In the electronics industry, various electronic components have been designed to incorporate a "getter" to chemically combine with the residual traces of gas in the component. A getter is also used to maintain the purity of the vacuum in evacuated electronic components. This is a less expensive means of protecting the electronic component than using vacuum drawing equipment to completely evacuate an electronic component. Materials commonly employed as getters in electronic components are barium and barium alloys such as barium-aluminum alloys. Barium and barium alloys are particularly suited for this use because these materials are sufficiently stable to permit safe handling during the assembly of the electronic component and yet are sufficiently reactive to effectively tie up the residual gases. Barium is the most widely used active metallic material for flash-type getters in electronic components.
Bulk-contact getters are used in higher temperature environments in limited volumes where a getter flash is impractical. The bulk getters, to be effective, must run hot, but are not flashed. Metals, or mixtures of metals used in bulk getters include thorium, titanium, cesium, zirconium, uranium, tantalum, hafnium, niobium, lanthanum, or mixtures of rare earth elements such as misch metal.
In light of the foregoing, it has been found desirable to minimize water, water vapor and other gases reactive with the cladding within the interior of the fuel element throughout the time the nuclear fuel is used in the operation of nuclear power plants. One such approach has been to find materials which will chemically react rapidly and combine with or absorb water, water vapor and other reactive gases to eliminate these from the interior of the cladding. While several getters for water and water vapor have been found, such as the zirconium-titanium getter set forth in U.S. Pat. No. 2,926,981, it has remained desirable to develop a getter having equal or greater rapidity of reaction with moisture and gases, and having the feature of rapidly reacting at temperatures encountered in nuclear fuel element fabrication.
It has been determined that in one preferred practice a getter in particulate form should be held in a container which will insure retention of not only the original particles of the alloy but also any reaction products of the alloy which could have much smaller average particle size. It has also been determined that the container for holding the alloy in particulate form should be easy to load, should be capable of being fabricated to given dimensions within close tolerances and should be relatively resistant to deformation during handling.